let KEYCODE_MEDIA_PLAY_PAUSE answer/hangup calls

By the Android USB headset spec:
https://source.android.com/devices/accessories/headset/usb-headset-spec



The Android key KECODE_MEDIA_PLAY_PAUSE should accept, reject, and end
calls when pressed appropriately.

This behavior is the same as the HEADSETHOOK behavior.
This updates the button handling code to match the spec.

Test: answered, muted, and hung-up call with USB headset adhering to spec.

Bug: 36070414
Change-Id: Id575d8b6178b76985a883e987908cf8c445c7bdf
Signed-off-by: Andrew Chant <achant@google.com>